In water electrolysis cells with alkaline electrolyte, the electrode reactions may be written as: EQU 2H.sub.2 O + 2e .fwdarw. H.sub.2 + 2OH.sup.- ( 1)
for hydrogen evolution, and EQU 4OH.sup.- .fwdarw. O.sub.2 + 2H.sub.2 O + 4e (2)
for oxygen evolution, the net cell reaction being EQU 2H.sub.2 O .fwdarw. 2H.sub.2 + O.sub.2 ( 3)
reaction (1) occurs at a standard potential of -0.83V, whilst the standard potential for reaction (2) is 0.4V. Clearly for an ideal electrolysis cell of this type, with perfect electrocatalysis and minimal resistance of electrodes and electrolyte, electrolysis should begin close to 1.23V (25.degree. C.). Similarly, in a hydrogen-oxygen fuel cell with alkaline electrolyte reactions (1) and (2) are reversed, the anode reaction is hydrogen oxidation whilst the cathode reaction is oxygen reduction; the net cell reaction is water formation. An ideal fuel cell of this type would produce power close to 1.23V (25.degree. C.).
In practical systems, however, the electrocatalysis is not ideal and electrolytes and electrodes have a finite resistance. Nevertheless by use of heavy loadings of platinum metals in optimized electrode structures, close spacing of electrodes, and other technological innovations, high energy efficiencies (above 70%) have been obtained for fuel cells and electrolysis units even for high power densities approaching 0.5W/cm.sup.2 of electrode surface.
In previous years, much attention has been given to synthesis of non-noble electrocatalysts for replacement of platinum metals in fuel cell and electrolysis cell applications. Considering the hydrogen consuming electrode in alkaline solution, an outstanding example of electrocatalyst development was the work on nickel boride by Jasinsky (W. Vielstich; Fuel Cells, Publ. John Wiley, 1970; Ch. 4, p. 171). This material, in the form of a porous conductive powder with a high surface area, was prepared by reducing a solution of a nickel salt in water using sodium borohydride. The boride electrocatalyst showed good stability and activity for hydrogen oxidation, the overvoltage was also low for hydrogen evolution. For oxygen reduction in alkaline solution mention may be made of the work of Kordesch (Ibid p. 178). On carbon or graphite, the oxygen reduction reaction does not go to completion, but instead produces hydrogen peroxide, present in alkaline solution as the perhydroxyl ion, HO.sub.2.sup.-. EQU o.sub.2 + h.sub.2 o + 2e .fwdarw. HO.sub.2.sup.- + OH.sup.-( 4)
reaction (4) proceeds at a standard potential of -0.08V, several tenths of a voltage below the standard potential of the full 4-electron oxygen reduction reaction (compare equation (2)); this would result in low fuel cell efficiencies. Kordesch catalyzed his carbon electrodes with mixed oxides of the transition metals. Some of these oxides, notably spinels of general formula AB.sub.2 O.sub.4 where A and B are metal ions, e.g., A=Co, B=Al, showed pronounced activity for hydrogen peroxide chemical decomposition. EQU 2HO.sub.2.sup.- .fwdarw. 2OH.sup.- + O.sub.2 ( 5)
and could work in concert with the 2-electron reaction (J. R. Goldstein et al., J. Phys. Chem. 76, 3646 (1972). By this means, the 4-electron reaction was made to go more or less to completion and the electrode performed quite efficiently, even when operated on air. The stability and performance level of these electrodes implied that other spinel oxides might prove useful electrocatalysts also.
In studies on composite electrodes for oxygen reduction akin to the Kordesch types, Goldstein (et al., J. Phys. Chem., 76, 3646 (1972)), investigated the graphite/cobalt-iron oxide spinel electrocatalyst system. They found (J. R. Goldstein et al., J. Catalysis, 32, 452 (1974)) that for the cobalt-iron oxide spinel system alone the composition Co.sub.1 Fe.sub.2 O.sub.4, which is a cobalt ferrite possessed the highest intrinsic activity for hydrogen peroxide decomposition in alkaline solution, and great stability under such conditions. When prepared in high surface area form by co-precipitation, the ferrite mixed with graphite showed good electro catalytic activity for oxygen or air reduction in the same way the Kordesch systems did. However, cobalt ferrite alone (with no graphite present), possessing poor conductivity, showed little electrocatalytic activity.